Optical communication systems typically handle optical pulses with frequencies in the range 2 to 10 gHz with the next generation of systems being expected to handle signals with frequencies up to 40 gHz. In existing systems, logic functions are performed on the pulsed signals as part of regeneration, error detection and clock recovery processes for example, it being typically necessary to convert the optical signals to electrical signals and to reconvert electrical signals to optical signals after performing logical operations and other processing.
The tendency for higher frequency signals means that it is increasingly desirable to avoid such conversion to electronic signals because of the difficulty and expense of high frequency electronic processing.
A number of optical logic devices and methods exist for performing logical operations without conversion to electrical signals but hitherto have proved unsatisfactory in the context of the above communication systems.
It is known from U.S. Pat. No. 4,764,889 (Hinton et al) to provide optical logic arrangements with self electro-optic effect devices for use in an optical digital processor. Control light beams are directed onto electro-optic devices in two dimensional arrays such that elements of the arrays have eight transmission or reflection characteristics which are optically controlled. Such arrangements are however suited to relatively low frequency parallel processing rather than logical functions between signals confined within waveguides.
Berthold discloses in U.S. Pat. No. 4,262,992 optical logic elements which operate on the principle of constructive and destructive interference between light beams, the refractive index of certain waveguides being controlled by applied voltages to produce relative phase differences in the propagated signals. A necessary condition for operation however is that optical signals are carried by input lights beams which are coherent, in phase and of equal amplitude which will seldom be the case in a practical communication system.
It is known from U.S. Pat. No. 4,932,739 (Islam) to provide logic functions which utilise soliton trapping between two optical signals propagated in a birefringent fibre. The technic however relies upon the use of frequency filters and polarisers. A further disadvantage is that, where a logic function is to be performed between two optical signals, both optical signals must be in the form of pulses having similar characteristics. Since the soliton trapping effect is associated with very long fibre lengths, it is anticipated that the lengths of fibre required would make use of such techniques impracticable.
It is known from Idler et al (IEEE Photonics Technology Letters, Vol. 8, No. 9, September 1996--"10 Gb/s Wavelength Conversion with Integrated Multiquantum-Well-Based 3-Port Mach-Zehnder Interferometer") to provide inversion of a single optical signal in addition to wavelength conversion by means of a Mach-Zehnder interferometer in which semiconductor optical amplifiers are utilised to set an interference condition between optical components of an input signal transmitted through first and second arms of the interferometer. A continuous wave optical signal propagated equally through the first and second arms is recombined to form an output signal which is modulated according to the interference condition and a pulsed optical signal is counter-propagated through only one of the arms so as to modulate the phase of one of the component signals by cross-phase modulation due to the non-linear characteristics of the semiconductor optical amplifier in that arm.
There remains a need for a practical technique for performing optical logic in an optical communication system.